1. Field of the Invention
The present invention relates to a shoe press for treating of a material web. The shoe press may include a press shoe, having a concave-shaped face, to be pressed against an opposing element, e.g., a counter roll, to form a nip of a predetermined longitudinal span. A fluid impermeable press jacket and a felt belt may be interposed between the press shoe and the opposing element. A plurality of support elements may include cylinder/piston devices supported on a carrier. The press shoe may move on the pistons of the support elements and may be carried to permit a tilting movement both in the web travel direction and lateral to the web travel direction, and may be supported in a web travel direction on a stop for absorbing horizontal forces acting on the press shoe.
2. Description of Background Information
A shoe press of the type similar in general to the above-described shoe press is disclosed in EP 0 345 501. According to EP 0 345 501, the press shoe rests so that it can move freely on the pistons of a number of cylinder/piston devices. A stop is provided behind the press shoe, i.e., with respect to a web travel direction, and the press shoe is pressed against the stop by frictional forces acting between the material web and the press jacket during operation. The invention of EP 0 345 501 strives to position the press shoe parallel to the axis of the counter roll.
However, in the shoe press of the prior art, the desired parallel arrangement is only achieved if a resultant pressure centrally engages the press shoe. In a normally selected pressure progression curve in the nip, pressure increases in the web travel direction, and the resultant pressure acts on the press shoe in an eccentric manner. This leads to a tilting motion, and a tilting of the press shoe with respect to the axis of the counter roll, as a result of a more intense compression of the felt and material web on an outlet end of the press shoe than on an inlet end of the shoe press.
As a result of the tilting motion, a distance between the press-shoe and the stop changes. If, e.g., the pressure is increased, the press shoe tilts such that the press shoe is lifted up from the stop on its outlet or nip outlet end. However, if the press shoe is resting against the stop, and the resultant pressure acting on the press shoe is tilted in an opposite direction in accordance with a decreasing eccentricity, which can occur, e.g., due to a reduction of the linear force or at a constant linear force with a reduction of the compressibility of a newly inserted felt, e.g., when being broken in during the first hours of operation, then jams and a high degree of sticking occur between the stop and the press shoe, between the press shoe and the pistons, and between the pistons and the piston guides.
The clamping forces can become so high that considerable wear must be contended with and the attendant friction forces can undesirably change the pressure distribution in the web travel direction acting in the nip. In particular, if the clamping does not occur over an entire width of the machine, varying pressure distribution curves may result over the width of the machine. This may, in turn, have undesirable consequences on the uniformity of the paper quality.
In the shoe press of the prior art, it is considered disadvantageous that the initial position of the press shoe can be crooked or bowed before the start of loading and then remain that way when loaded because the disadvantages mentioned with regard to paper quality result.